The cost per watt of electricity generated directly from sunlight by photovoltaic devices, i.e., solar cells, will have to be drastically reduced before such a system is economically competitive with conventional sources of energy. Two paths to low cost generation of electricity directly from sunlight are actively being investigated: first, low efficiency but extremely low in cost solar devices such as amorphous silicon solar cells and secondly, high efficiency but more expensive solar devices fabricated from group III-V compounds, e.g., gallium arsenide (GaAs) solar cells.
Gallium arsenide solar cells which exhibit efficiencies of 18.5% to 23.0% have been fabricated by Woodall et al, J. Appl. Phys. Lett., Vol. 30, No. 9, pp. 492-493 (1977) and James et al, Appl. Phys. Lett., Vol. 26, No. 8, pp. 467-470 (1975). However, gallium arsenide solar cells require a passivating layer to increase the overall efficiency of the solar cell and reduce the surface recombination losses and lower the surface recombination velocity of the device. Gallium arsenide solar cells incorporating aluminum gallium arsenide (AlGaAs) as the passivating layer are known. However, the AlGaAs compounds with large AlAs concentrations are chemically unstable to moist ambient weather and thus the long term reliability of such solar cells is questionable. In addition, the AlGaAs passivating layer is applied by liquid phase epitaxy, a process which greatly adds to the cost of GaAs solar cells.
A compound which can be applied to the surface of GaAs by vapor phase epitaxial deposition instead of liquid phase epitaxial deposition would simplify the production process and reduce the cost of a GaAs containing solar cell. Indium gallium phosphide (InGaP) has been shown to be an effective surface passivator for gallium arsenide containing semiconductors by C. J. Nuese in J. of Electronic Materials, Vol. 6, No. 3, pp. 253-293 (1977). InGaP is inert to moist ambient weather and can be applied by vapor phase epitaxial deposition. However, the compound absorbs all photons with wavelengths shorter than about 650 nanometers. Since about 40-50% of the energy from solar radiation has wavelengths less than 650 nanometers, the absorption properties of InGaP are unacceptable for terrestrial solar cells.
Thus, it would be highly desirable to find a passivating layer for a solar cell comprising gallium arsenide which could be applied by the cost effective process of vapor phase epitaxy, exhibit resistance to ambient moisture, and be transmissive to photons with wavelengths shorter than about 650 nanometers.